Carpenter Bee (Xylocopa micans) Risk Indifference and a Review of Nectarivore Risk-sensitivity Studies

Abstract

SYNOPSIS. This paper presents new results of risk-sensitive foraging studies of the carpenter bee, Xylocopa micans, and reviews the work to date on risk sensitivity in nectarivores. In the field, nectarivores choose among alternative food sources (flowers) that differ in the variabilities of their nectar rewards. In the lab, the foraging situation for carpenter bees was experimentally simplified by offering the bees a choice between either or artificial flowers. The two flower types differed in their variabilities but offered the same expected shortterm rates of net energy gain to test the predictions of the short-term rate maximization mechanism. Foragers were tested under two energy budget conditions, hungry and well-fed, to test the predictions of the z-score model. Individual carpenter bees were indifferent to variability in both nectar volume and nectar sugar concentrations, and their risk-indifference was unaffected by energy budget. These findings of risk indifference support neither the variance discounting nor the z-score model of risk sensitivity. Since the low and high variance flower types are equivalent for carpenter bees in short-term rate of energy gain, there can be no selection on carpenter bees to be sensitive to variability based on differences in rate of gain. Studies of risk sensitivity in honey bees and bumble bees using variance in nectar concentration support this contention. These findings are compared with other nectarivore risk sensitivity studies in order to highlight the most likely mechanisms underlying aversion to variation in nectar rewards (short-term rate maximizing, the Weber-Fechner law of perception and learning non-empty flowers) and to suggest future research in the interplay of these three mechanisms.